The Internet Protocol (IP) Multimedia Subsystem (IMS) is a well-known Third Generation Partnership Project (3GPP) standard allowing sessions to be set up between two or more parties for a broad variety of services such as voice or video call, interactive messaging sessions or third party specific applications. A few common enablers are defined by 3GPP for common usage among all these services. Examples on such enablers are capability discovery and subscribing to conference events to be used by for example an ad-hoc voice call conference for clients/devices to identify who leaves and enters the conference.
The protocol chosen by 3GPP is the Session Initiation Protocol (SIP). SIP provides a mechanism for the registration of user agents and for setting up multimedia sessions. The SIP REGISTER method enables the registration of user agent's current location and the INVITE method enables the setting up of a session.
SIP does not only allow the registration of users and the set-up of connections. Modification of sessions is also possible, such as adding media to a session or new user agents joining multicast sessions such as video conferencing. In order to assist a user agent in discovering the capabilities of other user agents and events that occur within the session, a number of other SIP methods have been developed. The SIP OPTIONS method enables a user agent to query another user agent or proxy server about its capabilities. This allows a client to discover information about supported methods, content types, extensions, codecs etc. The SUBSCRIBE method is used to request asynchronous notification of an event or set of events at a later time. It is complemented by the NOTIFY command by which these notifications are enacted.
Although SIP enablers such as SUBSCRIBE and OPTIONS are powerful, they are also resource intensive and use valuable signalling bandwidth. IMS separates the control functions and the user traffic, and resource intensive methods such as those used by SIP can place a strain on resources dedicated to signalling within the IMS. For example, signalling must be routed through the home network. Acknowledgement and retransmission messages as well as provisional responses are incorporated in SIP which increase the amount of signalling required. In addition, new logic and commands for networks can only be developed through the standardisation process.